1. Field of the Invention
This invention relates to an image processing apparatus such as, a microfilm scanner and a copying device and to a method for density adjustment of a copied image. More particularly, this invention relates to an apparatus and a method which are adapted to permit automatic modification of the amount of change in density per unit step during the formation of an image in conformity with the contrast of an original image (inclusive of a film or an original document).
2. Description of the Prior Art
The microfilm scanner in popular use to date, for example, performs a pre-scan on a given film as an original image and, based on the outcome of the pre-scan, determines an exposure level for the formation of a copied image of desired density for the purpose of obtaining a copied image of high quality.
Incidentally, the amount of change in density per step of the exposure level is generally fixed at a constant value. The exposure level necessary for obtaining a copied image of desired density, however, is possibly variable with the kind of the film bearing a photographed original image, such as a negative film and a positive film. To cope with this variation, some devices have been adapted to permit variation of the amount of change in density per step of the exposure level in conformity with the kind of film as disclosed by the invention of U.S. Pat. No. 4,591,264, for example.
Although the conventional image processing apparatus embodying this principle is so adapted as to permit modification of the amount of change in density per step of the exposure level in conformity with the kind of film, the amount of change in density per step of the exposure level is fixed at a constant value for similar films. If the amount of change in density per step is adequately set for films of high contrast, for example, it will cause the films of low contrast which are similar to the films of high contrast, to incur the following inconveniences.
When a film (1) having a contrast of 1.0 and a film (2) having a contrast of 0.3 are selected to be darkened each by one step, where the amount of change in density per step is fixed at 0.1, for example, the one step in the film (1) relative to the contrast of film equals the change in density shown below:
0.1/1.0=1/10
The density of the copy, therefore, turns out to be a gradually darkening output as compared with the copy density existing before the setting of the exposure level.
Meanwhile, the one step in the film (2) relative to the contrast of film equals the change in density shown below:
xe2x80x830.1/0.3=1/3
The density of the copy, therefore, turns out to be a suddenly darkening output as compared with the copy density existing before the setting of the exposure level.
As a result, the operator inevitably incurs an inconvenience of finding no way of setting the density at a medium value between step 0 and step 1 in spite of the most likely possibility of demanding an exposure level equivalent to this medium value.
When this situation is completely reversed, the possibility of the operator likewise experiencing an inconvenience is undeniable. Specifically, this is the case in which the amount of change in density per step turns out to be adequate for films of low contrast.
When a film (3) having a contrast of 0.3 and a film (4) having a contrast of 1.2 are elected to be darkened each by one step where the amount of change in density per step is fixed at 0.03, for example, the one step in the film (3) relative to the contrast of film equals the change in density shown below:
0.03/0.3=1/10
The density of copy, therefore, turns out to be a gradually darkening output as compared with the copy density existing before the setting of the exposure level.
Meanwhile, the one step in the film (4) relative to the contrast of film equals the change in density shown below:
0.03/1.2=1/40
The density of copy, therefore, turns out to be an extremely feeble darkening output as compared with the copy density existing before the setting of the exposure level.
As a result, the operator is compelled to make a change involving a large number of steps. Since the number of steps available at all for the change is limited, he inevitably confronts a difficulty contrary to the hardship mentioned above, i.e. an inconvenience of finding no way of elevating the density to a required level.
The occurrence of such inconveniences as mentioned above leads to disruption of the uniformity of change in density per step of the exposure level between original images abounding in contrast and original images lacking contrast to a point where the operator will experience difficulty in adjusting the exposure level for the purpose of obtaining copied images of desired density.
It is an object of this invention to provide an image processing apparatus which adjusts the change in density per step of the exposure level for a given original image to suit the contrast of the original image, no matter whether the contrast may be high or low.
To fulfill the object mentioned above, the image processing apparatus according to the present invention comprises an image reading means for reading out an original image and outputting an electric signal corresponding to the density of the original image so read out, contrast calculating means for calculating the contrast of the image based on the electric signal outputted by the image reading means, density setting means for setting desired density within a prescribed adjustment width, adjustment width setting means for setting the adjustment width of the density setting means based on the contrast calculated by the contrast calculating means, and density adjusting means for controlling the electric signal outputted from the image reading means in accordance with the value of density set by the density setting means and thereby adjusting the density.
By adopting the construction described above, it is made possible to set the adjustment width of copy density based on the contrast of the original image read out and use the density adjusting means effectively for implementing adequate density adjustment for an original image of any degree of contrast.
It is another object of this invention to provide a method for effecting density adjustment in an image processing apparatus which enables the change in density per step of the exposure level to be adjusted to suit the contrast of an original image.
To fulfill the object mentioned above, the method for density adjustment according to the present invention is used in an image reading apparatus adapted to have a prescribed adjustment width divided into a plurality of steps and provided with density setting means capable of setting amounts of density adjustment stepwise depending on the number of steps to be set, which method comprises a first scan step for reading out an original image by the use of image reading means and outputting an electric signal corresponding to the density of the original image, a histogram forming step for forming a density histogram based on the electric signal corresponding to the density obtained in the first scan step, a contrast calculating step for calculating a contrast based on the density histogram, a density adjustment amount setting step for enabling density setting means to set the amount of density adjustment in one step based on the contrast, a density adjusting step for controlling an electric signal outputted from reading means in accordance with the amount of density adjustment set by the density setting means and thereby adjusting the density, and a second scan step for reading out the original image again and outputting an electric signal using the state of density setting.
The method described above allows the density of an image of a copy to be invariably adjusted to an adequate level because it commences the second scan after detecting the trend of the density of an original image in the first scan, forming a density histogram in response to this trend, discerning the contrast of the image from the density of the blank base part and the image formed part, and then setting the amount of density adjustment in one step based on the contrast.